The prolific use of computers and other high speed electrical processing equipment has generated an ever-increasing number of applications for magnetic recording media. For example, magnetic recording disks, commonly referred to as hard disks, are used to record and store large amounts of electronic data and are widely utilized throughout the computer industry. Furthermore, other types of magnetic recording media are utilized for similar recording and storage applications.
Magnetic recording media, such as a hard disk, generally comprise a series of film layers deposited upon a substrate base with a magnetizable recording layer positioned as one of the topmost layers. The recording layer is usually a thin polycrystalline film comprising a plurality of metal crystals oriented in a number of different directions. Metal crystals have a defined crystal structure or shape. The individual crystals orient themselves in a particular direction within a metal layer or film to define the crystal texture of the film. The magnetic properties of the film layer, and hence the recording properties of that layer, are dependent upon the crystal texture of the metal film and more specifically upon orientation of the individual crystals with respect to the surface plane of the film.
Polycrystalline cobalt-based alloys are the most common materials used for thin film magnetic recording media. Such cobalt-based alloys include, for example, cobalt nickel chromium (CoNiCr), cobalt platinum chromium (CoPtCr) and cobalt chromium tantalum (CoCrTa), among others, which are sputter deposited onto the substrate. As an illustrative example, an aluminum magnesium (AlMg) base substrate might be plated with a layer of nickel phosphorous (NiP) for hardness and machining purposes. The first thin film layer deposited would then typically be a chromium (Cr) underlayer which determines the crystal texture of the upper layers as described further hereinbelow. Next, the magnetically active cobalt-based recording layer is sputter deposited directly upon the chromium underlayer. A protective overcoat might then be applied over the cobalt-alloy recording layer.
The crystal structure of cobalt is generally that of a hexagonal closed-packed crystal or HCP crystal, as opposed to a body-centered cubic (BCC) or cubic close-packed (CCP) crystal structure. Because of the different atomic arrangements in certain planes and directions within an individual crystal or unit cell, the properties of a crystal cell will vary depending upon the direction or plane along which the property is measured. This is referred to as anisotropy. Cobalt has high crystalline anisotropy, and, as a result, cobalt-alloy recording films have high coercivity and high magnetic remanence which are both desirable properties for magnetic recording media. Coercivity is defined as the measurement of the strength of a reverse magnetic field which is necessary to erase stored data and randomly orient the magnetization of the various crystal structures so that they cancel one another's effects. That is, high coercivity means that a high reverse field must be applied to the recording surface to erase the stored data. Remanence, on the other hand, refers to the resistance offered by the magnetized material which prevents random orientations of the magnetized crystals once the magnetizing field has been removed. That is, it refers to the ability of the film to maintain the stored data once it has been recorded. When the storage media maintains the magnetization in the direction of the original recording field, a residual magnetization (i.e., remanence) is created whereby the material tends to act like a permanent magnet.
Presently, there are two basic types of recording modes. The first type of recording mode is referred to as longitudinal recording and is the basis for most magnetic recording media that is used today. In a longitudinal recording mode, the magnetization of the magnetic storage layer and the magnetization of the crystals in the storage layer, is accomplished in a direction parallel to the surface plane of the sputter deposited thin film recording layer. Therefore, longitudinal recording requires that the magnetic easy axes of the crystals, which are those axes most easily magnetized, be oriented in the film surface plane. This property is termed in-plane anisotropy.
The second recording mode is perpendicular recording. Perpendicular recording holds promise for use in high storage density recording media and is likely to be used more widely in the future. In perpendicular recording, the magnetization is accomplished in a direction normal to the surface plane of the film layer, and therefore, the magnetic easy axes of the crystals making up the storage layer are preferably oriented normal to the film layer surface. This type of orientation is called perpendicular anisotropy and is preferred for perpendicular recording.
Additional crystal orientations also exist in a recording film layer, and the magnetic easy axes of various crystals may be angled slightly with respect to the plane of the film surface between 0.degree. (in-plane anisotropy) and 90.degree. (perpendicular anisotropy). Actual magnetic recording media layers typically have a mixture of both in-plane and perpendicular anisotropy. In the case of longitudinal recording, a perpendicular anisotropy component is useful to suppress media noise, and from the point of noise suppression, it is desirable to "tune" the film anisotropy as necessary to take advantage of the noise reduction properties of the film. Therefore, manipulation of the various orientations of crystals, i.e., manipulation of the crystal texture, in a recording film layer is desirable.
In both longitudinal and perpendicular recording media utilizing sputter deposited cobalt-based alloys, the underlayer film will generally consist of one of chromium (Cr), vanadium (V) and tungsten (W). Cr, V and W have body-centered cubic crystal structures (BCC). Depending upon sputtering conditions such as the deposition pressure and substrate temperature, certain crystal planes of the BCC structure are most commonly found to be parallel to the film surface plane. BCC crystal structures have planes designated by three-digit Miller indices. As an example, in a thin film of BCC chromium, the (200) and (110) designated planes are most commonly found parallel to the film surface plane. After the underlayer is deposited, the cobalt-alloy recording layer is then sputter deposited on top of the underlayer. The underlayer texture is critical because it has been found that the crystal orientation of the underlayer film may be used to select the texture of the recording layer.
More specifically, cobalt and chromium have several interplanar atomic spacing values that are nearly identical, and a particular planar orientation of the crystals in the chromium underlayer establishes a specific planar orientation of the crystals in the upper cobalt-alloy recording layer which is deposited over the underlayer. The texture of the cobalt-alloy layer controls the recording properties of the layer, and thus, it is desirable to control the texture of the underlayer in order to establish the recording layer texture and achieve optimum recording characteristics. In other words, it is desirable to control the growth of the crystals in the sputtered underlayer to achieve a particular texture in the sputtered recording layer depending upon the type of recording for which the recording media is to be used, i.e., longitudinal or perpendicular recording.
The texture of the underlayer has been controlled in the past by varying the sputter deposition temperature, the deposition pressures and/or the thickness of the sputtered layer. For example, elevated deposition temperatures typically promote the growth of a chromium film on a Ni-plated AlMg substrate with the (200) plane parallel the film plane. Lower sputter deposition temperatures usually result in (110) plane chromium films. However, even when the films are deposited using identical sputtering pressures and temperatures, the magnetic properties of the films, when grown on different types of substrates, will vary widely. Furthermore, there is evidence that elevated deposition temperatures, such as those needed to promote the growth of a specific chromium underlayer texture and subsequent magnetic recording layer texture, has negative effects on the quality and mechanical properties of the layers and on the sputtered carbon overcoat that is commonly employed in the industry to protect the magnetic recording film.
Therefore, it is desirable to have a method, and particularly a low temperature deposition method, which may be used to control and modify the texture of the underlayer film in magnetic recording media. Further, it is desirable to control the texture of the magnetic recording layer to achieve the desired magnetic properties of the recording layer.